1. Field of the Invention
The present invention relates to an image-casting control method of a cathode ray tube image display device such as for a television picture tube or data terminal device, and particularly to an image-casting control method of a cathode ray tube image display device having a cold cathode.
2. Description of the Related Art
The fundamental operation of a cathode ray tube involves the use of electron emission from electron sources, focusing, acceleration, and deflection to cause excitation of fluorescent material on a screen by electron beams and, finally, emission of light. Cathode ray tubes of the prior art use an thermionic source in the electron source that takes advantage of development through thermionic emission.
The thermionic source generally employed in a cathode ray tube obtains thermions by using a heater to heat a cathode pellet made up of oxide mixtures such as barium, calcium, and strontium. An electron gun is constructed by combining these thermionic sources and a plurality of electrodes, and various functions such as the control of the amount of electron emission as well as focusing and acceleration of the electron beam can be achieved by applying a prescribed voltage to each of the electrodes.
When a power source is turned on to operate the thermionic sources when in a halted state, about 5 seconds is necessary for the temperature of the thermionic source to increase from room temperature to a prescribed temperature (for example, about 750xc2x0 C.) that allows electron emission.
Conversely, when the thermionic sources are stopped while in operation, several seconds are necessary before the temperature drops to the point at which electron emission ends (about 500xc2x0 C.) even when power to the heater is cut, and thermionic are therefore emitted from the cathode during this interval, and an electron beam may be irradiated toward the screen.
In a display device using a cathode ray tube, when the power source is turned off, a high-capacity smoothing capacitor is used in the high-voltage power source that supplies a positive high voltage to the screen to stabilize the high, direct-current voltage. As a result, even though the power source is cut off, the high-voltage output is not immediately interrupted but rather, drops gradually. In comparison, horizontal and vertical deflection circuits that do not use thermions fall rapidly, and this results in a state in which the electron beam undergoes no deflection and continues to irradiate for a period of time concentrated at only the central portion of the fluorescent screen, and this state results in the problem of a remaining spot that can cause xe2x80x9cstickingxe2x80x9d or burning of this portion of the fluorescent screen. Several image output circuit control methods, generally referred to as xe2x80x9cspot killers,xe2x80x9d have been disclosed as a means of avoiding this type of phenomenon after electron beam emission has reached a normal state.
As shown in FIG. 1, Japanese Patent Laid-open No. 231567/91 discloses a method of preventing a remaining spot by which capacitor 24 is charged after the power source is turned on and a steady state is achieved, and when the power source is turned off, the fall of voltage at the +B terminal is sensed, whereby spot-killer circuit 22 is activated, the voltage of charged capacitor 24 is used to change the bias of image output circuit 23 to the direction of flow of the beam current, and the high voltage charged in cathode ray tube 21 is discharged before the horizontal and vertical deflection circuits are stopped.
Japanese Patent Laid-open No. 245178/88 discloses a method for a case in which a circuit system as shown in FIG. 2 is digitized. In a case in which synchronizing deflection circuit 39 is digitized, when the power source is turned off, the fall in voltage is sensed, and system reset circuit 32 is caused to operate, whereby synchronizing deflection circuit 39 is halted instantaneously and horizontal and vertical deflection are no longer performed. To prevent this from happening, a system reset signal causes on-screen blanking transistor 33 to turn on, and this in turn causes image output transistors 35, 36, and 37 of image output circuit 34 to turn off instantaneously, whereby the cathode ray tube (not-shown) enters a blanking state in which the bias of the cathode is cut off and electron emission is halted. The electron beam can therefore be prevented from entering a static spot state. The on-screen blanking used in this case is a function used for blanking the background portion of letters displayed on the screen.
A cathode ray tube that employs a cold cathode such as a field emission cold cathode, which is a quick-acting electron source, as the electron source can dispense with the heating of the cathode by a heater, which is required in a hot cathode. The principle of electron emission in a field emission cold cathode is the emission of electrons from a solid to a vacuum brought about by a quantum-mechanical tunnel effect when a strong field of 107 V/cm or more is impressed to a solid surface.
FIG. 3 shows one example of the structure of a field emission cold cathode. A high field can be obtained by applied voltage between a sharp needle-like emitter cathode 15 having a tip radius on the order of 100 nm and gate electrode 14 arranged approximately 0.5 to 1 xcexcm away from the emitter, thereby creating field concentration at the tip of emitter electrode 15. A multiplicity of emitter-gate constructions of this type formed on a substrate 12 and connected in parallel can be used to lower the applied voltage so as to obtain a prescribed current.
The degree of sharpness of the tip of emitter electrode 15 and the isolation characteristic between emitter electrode 15 and gate electrode 14 are key conditions for maintaining the electron emission characteristic of a field emission cold cathode.
As described hereinabove, the size of each emitter and gate is extremely small, and a large number of structures can be integrated in a small area. As shown in FIG. 4, one example of the electron emission characteristic with respect to the applied voltage shows that electron emission starts from approximately 30 V and rises sharply.
However, the above-described prior art has the following problems. First, when power is turned on to a display device using a cathode ray tube that employs a quick-acting electron source such as a field emission cold cathode, the electron beam irradiates only the center of the screen, thereby causing burning of the fluorescent screen. The reason for this is that electron emission in a field emission cold cathode begins immediately upon applying voltage that meets prescribed electron emission conditions. When the power source of the device is turned on, electron emission thus begins before the horizontal and vertical deflection circuits have risen sufficiently, whereby the electron beam irradiates the center of the screen without undergoing deflection.
The second problem is the destruction of element structures caused by sputtering resulting from the bombardment of positive ions and the degrading of isolation characteristics caused by re-adhesion of sputtered particles. The electron beam ionizes residual gas molecules or gas molecules generated by irradiation of the screen within the cathode ray tube, and positive ions thereby generated are accelerated in the direction opposite that of the electron beam. When the electron beam is being deflected in a stationary state, the paths of electrons and positive ions differ due to their difference in mass, and the positive ions therefore do not reach the electron source, but when the electron beam is directed straight ahead without being deflected, the positive ions bombard the electron source. As shown in FIG. 3, a field emission cold cathode involves-the input of voltage to a minute structure, and a field emission cold cathode is therefore extremely sensitive to bombardment by positive ions.
It is an object of the present invention to provide a method and device for preventing a remaining spot when power is turned on in an image display device incorporating a quick-acting electron source.
It is another object of the present invention to provide a method and device for improving the reliability and life of an image display device by preventing concentrated positive ion bombardment of the cathode in a device incorporating a quick-acting electron source.
The image-casting control method of the image display device of the present invention consists in delaying the start of operation of the electron source when the power source is turned on until the electron beam deflection system attains a steady operating state.
According to the present invention, in an image display device incorporating a quick-action electron source such as a field emission cold cathode, after the power source of the device is turned on, electron emission does not start until after the horizontal/vertical deflection circuit has risen sufficiently, thereby preventing burning of the fluorescent screen when the power source is turned on.
According to the present invention, despite ionization of gas molecules inside a cathode ray tube by the electron beam and generation of positive ions, the electron beam is deflected and the electrons and positive ions therefore have differing paths due to their difference in mass, and positive ions for the most part do not reach-the electron source, thereby allowing suppression of both damage to the element structures caused by sputtering due to bombardment by positive ions as well as deterioration of the isolation characteristic due to re-adhesion of sputter particles.
In addition, an image display device according to the present invention includes either a deflection current sensing circuit that drives an RGB output circuit or RGB pre-amplifier only when the deflection current flowing from horizontal/vertical deflection output circuits to the deflecting yoke is sufficient to enable the electron beam to adequately scan the screen; or
a system control circuit that, after the main power source of the device is turned on, drives the horizontal/vertical deflection output circuit and supplies a deflection current to the deflecting yoke, and, when a sufficient deflecting current is flowing to enable the electron beam to adequately scan the screen, drives an RGB output circuit or an RGB pre-amplifier.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with references to the accompanying drawings which illustrate examples of the present invention.